I can't get through a cave in the Orobas Fjords.

It's the cave with the book for my alchemist, near the fortress with the levers to disable the force fields at the start of this area. There's also one of the bandits in it (Ragon, I think). Coordinates ca. (9, 27) on this map (scroll down to correct map).

When I enter the map I find the body of a dead priest and a key nearby. No idea what the key does.
Slightly down the path there's a big group of bandits. The cave splits: The path to the left leads to a locked door with a gate. I can't get through the gate.
The right path leads to a force field. I can see what's behind it, but I can't get through.

I could use some help here.